A surface attachable electrochemical cell (electric double layer capacitor and battery) is used in a backup power supply for a clock function, a backup power supply for a semiconductor memory, and the like. In such compact electrochemical cells, the need for larger capacitance and larger current is reducing as progress is being made in the non-volatilization of semiconductor memories and the lowering of the power consumed by clock function elements. However, demands for resistance to reflow soldering, reduction in the attachment area, and the like are increasing due to environmental concerns and miniaturization of the attached devices.
When attaching an electrochemical cell, solder cream etc. is applied in advance to the portion of a print substrate that is to undergo soldering. The electrochemical cell is attached on this portion. Alternatively, a small solder ball (solder bump) is applied to the portion that is to undergo soldering after the electrochemical cell is attached on the print substrate. The print substrate on which the electrochemical cell is attached is then conveyed through a furnace having an atmosphere set at a high temperature so that the soldering portion becomes higher than or equal to the solder melting point (e.g., 200° C. to 260° C.). This melts the solder and solders the electrochemical cell. Thus, it is strongly required that the electrochemical cell have heat resistance and mechanical resistance relative to reflow soldering.
Further, in the prior art, an electrochemical cell includes a case with a round form like a coin or a button. In such an electrochemical cell, terminals and the like must be welded to the outer surface of the case in advance. Furthermore, space for the terminals must be provided on the print substrate. Therefore, miniaturization of attachment devices has resulted in a strong demand for reduction in the attachment area for the electrochemical cell.
The electrochemical cell of patent document 1 uses a hollow ceramic container (hereinafter referred to as hollow container) as the case, and electrodes and electrolyte are accommodated in the hollow portion (accommodation compartment) to answer the demand for heat resistance and reduction. A metal film arranged on the outer bottom surface of the hollow container is used as a terminal.
The terminals of patent document 1 include an inner terminal formed in the hollow portion and outer terminals formed on the outer surface of the hollow container, and the cathode arranged in the hollow portion and the outer terminals are electrically connected. Such terminals are formed in the following manner using a plate-shaped ceramic green sheet and a frame-shaped ceramic green sheet that configure the hollow container. Pattern printing with a material that is mainly based on tungsten or molybdenum having a high melting point is performed on the upper surface of the plate shaped ceramic green sheet. The frame-shaped ceramic green sheet is laminated on the plate shaped ceramic green sheet having the pattern, and the two sheets are sintered together at approximately 1500° C. The inner terminals and the outer terminals are both formed as a result.
However, the material forming the terminals is corroded by charge or discharge current when contacting electrolyte such as a liquid. As the corrosion proceeds, wire breakage may ultimately occur and hinder functioning.
Such problem is considered to be avoidable by relying on protection provided by a protective film formed on inner terminals to protect the inner terminals from electrolyte. The protective film uses materials mainly based on aluminum or carbon and causing less corrosion and is formed through any method of vapor deposition, sputtering, thermal spraying, injection, paste application, and the like. However, microscopic pin holes may form since the film is obtained by depositing particles in each of these methods. As a result, if the protective film does not have pin holes, satisfactory property would be realized over a long period of time. However, if the protective film has pin holes, electrolyte may gradually permeate into the protective film and ultimately corrode the inner terminals thereby causing wire breakage in the terminals. The production of pin holes may be reduced by increasing the film thickness of the protective film. However, this would increase the time required for film formation and cause the electrochemical cell to be expensive. Furthermore, the thickness of the electrode must be reduced by an amount corresponding to the thickness of the protective film. This would reduce the capacitance of the electrochemical cell.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-216952.